1. Field of the Invention
This invention relates to a method of growing a single crystal CaF.sub.2 film on an Si sunstrate to form an SOI (Silicon on Insulator) structure using CaF.sub.2 as an insulation layer.
2. Description of the Prior Art
An SOI structure comprising a single crystal insulation film formed on an Si substrate with a single crystal Si film formed thereon is known as being able to attain higher intergration, higher speed, and lower power consumption of a semiconductor integrated circuit.
CaF.sub.2 has a cubic crystal structure with a lattice constant of 5.46 .ANG. and a very good lattice mismatching ratio of 0.6% with Si. Thus, it permits growth of good single crystal CaF.sub.2 on and Si substrate. This may be used as an insulation film in SOI structure. Since evaporation of CaF.sub.2 in stoichiometric composition easily occurs under vacuum, the vacuum deposition method and the molecular beam epitaxial method are used for the growth of CaF.sub.2.
The growth of CaF.sub.2 (111) film on an Si(111) substrate (single crystal Si substrate with (111) principal plane) is reported, for example, in Japanese Journal of Applied Physics Vol. 22, No. 10, Oct., 1983, pages 1474-1481. According to this report, a CaF.sub.2 (111) film with farily flat surface morphology and good crystal quality grows on the Si(111) substrate at a substrate temperature of 600.degree. C. or higher. In this case, small triangular cracks of 10 nm high may be produced, but its production may be controlled by the optimization of the growth condition. In the crystal quality estimation by electron diffraction, a Kikuchi Line is observed in RHEED (Reflection High Engery Electron Diffraction) image, indicating high-quality crystal quality.
Thus, films satisfactory in crystal quality and flatness have been obtained, but they have drawbacks in using as an insulation film for forming an SOI structure comprising Si/CaF.sub.2 /Si structure as follows: A CaF.sub.2 (111) film grown at a substrate temperature near 700.degree. C. relatively has a chemical stability, but those grown at a substrate temperature lower than it are chemically unstable and easy to dissolve in an acid, alkali, or organic solvent, and are not adapted to the manufacturing process of a semiconductor device. In addition, since an Si(111) film grown on a CaF.sub.2 (111) film has many dangling bonds on the (111) plane, when an oxidation film for MOS device is formed, the interface level density of Si/SiO.sub.2 interface becomes too high to provide satisfactory characteristics of the MOS device.
In contrast, a CaF.sub.2 (100) film grown on an Si (100) substrate is chemically stable regardless of the growth temperature, permitting easy manufacturing process of a semiconductor device, the interface level density of Si/SiO.sub.2 interface is small, and is suitable to the manufacture of the MOS device.
However, the optimum substrate temperature range for the growth of CaF.sub.2 (100) film on the Si (100) substrate is limited to 500-600.degree. C., and it is known that the surface morphology of the growing film will become roughened having column structure. An improvement is desired.
FIG. 1 is an electromicrograph (SEM photograph) by a scanning electron microscope (SEM) showing surface conditions of a CaF.sub.2 (100) film grown several hundreds nm on a Si (100) substrate at a substrate temperature of 550.degree. C. It exhibits a roughened surface morphology of 20-30 nm square column structure as reported in the Journal of Vacuum Science Technology, pages 1026-1031, A4(3), May/Jun., 1986. It is a question to grow an Si (100) film on such a surface.
It is possible for improving the crystal quality and surface morphology to give rapid thermal annealing after the CaF.sub.2 (100) film has been grown as indicated in the Jornal of Electrochemical Society, pages 224-227, Vol. 133, 1, Jan., 1986. This literature reports that any CaF.sub.2 (100) film grown on an Si(100) substrate at any temperature of 300.degree. C. of higher is improved in crystal quality by annealing in argon atmosphere at 1100.degree. C. for 20 seconds, and the surface morphology is changed from rough surface to flat one as well.
The above annealing is effective to improve the crystal quality and surface morphology of the CaF.sub.2 (100) film. However, as reported in the literature, there is a problem in the control of the conditions such as temperaturue, time, and atmospheric gas in the annealing treatment. In addition, on the surface of CaF.sub.2 (100) film after annealing, ashler type cracks are apt to occur in the &lt;110&gt; direction. Such cracks not only deteriorate the quality of the Si film grown on the CaF.sub.2 (100) film, but also deteriorate the breakdown field of the CaF.sub.2 insulation film itself, leading to increased leak current and deteriorated insulation film characteristics.